The present invention relates to an alarm system. More particularly, the present invention relates to an atmospheric abnormality detection alarm system wherein a plurality of sensor terminals are separately connected to a single transmission line and are cyclically accessed by a main unit (central receiver). An accessed sensor terminal converts an analog signal (indicating smoke concentration, temperature, gas concentration or the like) to a digital signal which is then transmitted to the main unit. The main unit processes the digital signal to detect any atmospheric abnormality (e.g., fire or gas leakage) at this sensor terminal or malfunctioning of the sensor itself, and then displays the detected contents and generates an audible alarm.
In the conventional system of this type, when an abnormality is detected at a sensor terminal, the main unit indicates this sensor terminal by its address and generates an audible alarm in accordance with the alarm content. For example, maintenance personnel are made aware of the occurrence of a fire and its location. However, in practice, when maintenance personnel reach the detected site, it is very difficult to confirm the exact location where fire has broken out. This may disable early fire fighting, resulting in a big disaster.